iMac G5 17", power issues

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Joined: Apr 28 2009
Posts: 4

Hello all,

I recently picked up an iMac G5 that was described in the ad as "nonfunctional" I guess I got really lucky, because when I got it home I popped the back, reset the PMU (by removing the 2032 and pressing the internal power button, found through Google, please correct me if this isn't how to reset the PMU) and threw some RAM in. Wonderfully, it powered up. Now the strange part starts.

It wouldn't boot off of CD if you put in the disc and held 'c' it would only boot off CD once it got to the question mark Mac screen and you put the disc in, otherwise the Superdrive would kick out the disc. If you put the disc in after the question mark Mac screen, it would boot right up, happy as could be.

I threw an 80GB Western Digital SATA hard drive and 1GB of Kingston RAM in it (in addition to the 512MB of Samsung RAM that was already in there and booting perfectly) and then it started getting temperamental. It would start, Apple chime and all, with the back casing on, but it wouldn't get past that point, no matter how long you waited. No VGA would ever appear, and it, to the best of my observations, would not boot off the optical drive. Take the case off, and it would chime and VGA would pop up. At this point I started taking note of the length limitation on the RAM, and perhaps the fact that it could be shorting on the CPU heatsync. Measuring the RAM proved that it was slightly less than the maximum 31.75mm tall.

8 hours later: The iMac won't start up at all. Case on or off, the LEDs are as follows:
o---
Pressing the power button causes them to go to this, but only for a fraction of a second:
oo--
And then they return to:
o---

I've checked all the capacitors, both on the logic board and in the power supply, none of them are bulging and they all look as good as they would've brand new. I've modified and ATX power supply to give the iMac what it needs (aside from the 24V for the LCD inverter) and it powered up perfectly on that, however it did not go into disk target mode when holding down the 't' key. I replaced the ATX supply with the original, and it started up once, but this time without the third LED lit (LCD communication)

As it stands right now, neither the Apple PSU (which I've hotwired and tested all the voltages on, it's good) nor the modded ATX PSU (which I also measured voltages on) will power up the machine. If you press the power button, it will power the two left LEDs for a fraction of a second, and a faint clicking noise can be heard, and then the computer goes dead again.

I'm just unsure of how all this happened. No static discharges occurred, and the unit was unplugged when I was doing any work inside, aside from having the back case off to test the RAM issue. I know the symptoms sound like they may be blown/bulging caps, but all the caps are PERFECT. The power issue sounds like either a PSU or a cap issue, but given that I've tried with two PSUs and the same issue arises, I'm just unsure. Are there any capacitors on the opposite side of the board? I haven't removed the board yet to look. I've replaced blown caps on PCs before, so I'm sure none of these are bulging/blown, and I've done modifications to iMac G3s and PowerMac G4s before, and the ONLY other time I've experienced something similar to this was when I had a G4 "Mystic" and my +28V source was dead.

Attached picture is without the stock PSU inside the iMac, if you were wondering what was missing from it.

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Joined: Apr 28 2009
Posts: 4
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cwsmith's picture
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Joined: Oct 13 2005
Posts: 699
Possible culprits

• Bad 1GB stick of RAM? Try again with just the original 512, maybe again with a known-good stick.
• Wrong type of RAM? Should be PC3200, *not* PC2700 or PC2100.
• Debris in the RAM slots? Use compressed air to blow them out really well.
• Debris in the PSU connector? Again, compressed air.
• Debris in the LVDS (monitor) connector?
• Dead PRAM battery? How long did it sit in a box with no trickle power?
• Flaky AirPort card? Debris in the connector? I've seen it happen.
• Flaky modem card? Debris in the connector? Is it necessary?
• Flaky optical drive? Debris in the connector? You say it wouldn't boot to the DVD without jumping through hoops.

Could still potentially be a bad PSU -- can you borrow one from another machine and give it a try?

No caps on the underside of the MLB, but there is thermal paste between the GPU and the aluminum heatsink in the midplane. I wouldn't go removing the board until you've tried other troubleshooting steps, and make sure you clean up & replace the thermal paste.

Not only do I not see any blown caps in your photo, but this looks to be a remanufactured board: the original "bad" caps almost always had an X or + pattern on the venting end; yours mostly seem to have the K pattern, which indicates they were replaced, or the board is a later revision.

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Joined: Apr 28 2009
Posts: 4
• Tried it with the origina

• Tried it with the original stick, still no luck
• It is PC3200, not PC2700 or PC2100.
• The RAM slots look clean, however I will try to blow them out and see if that changes anything
edit: Couldn't see any debris, blowing them out changed nothing
• I've blown out the PSU and all the fans, but not the PSU connector, I'll give it a try.
edit: Blowing it out changed nothing
• I don't think there are debris in the LVDS connector as I did have VGA up and working before adding the 1GB stick.
edit: unscrewed the LVDS connector and blew it out, looked clean
• I haven't measured the PRAM battery, I'll replace it with a known good 2032
edit: measured the PRAM battery, 3.02V
• No idea if it's flaky, I'll disconnect it.
edit: Disconnected the airport extreme card, no change
• Again, no idea, disconnecting it will find out.
edit: Disconnected the modem, no change.
• No debris in the optical drive connector, perhaps the laser is dirty, I haven't blown out the drive itself.
edit: Blew out the connector and the drive, but have no way to test if this changes anything, as the machine still won't power on!

I'm going to try a PSU from another iMac 17" ALS (the 1.8Ghz model) tomorrow and see if that solves it. If so, then I'll just order a new PSU and move on.

I have no idea what the original manufacture date of the machine is. It isn't an iSight model, so I'm assuming that the machine was shipped before October 2005, but I honestly don't know the history of the machine, whether it's had a midplane replacement or not. I also noticed that many of the capacitors had the K tops, which I've never seen go bad. The PSU has the X tops, but none are swelling or blown.

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Joined: Apr 28 2009
Posts: 4
It turns out that a component

It turns out that a component on the midplane, an International Rectifier F7410 8-pin DIP is fried. I've order replacements, but they'll take 2-3 weeks to come in, so we'll see what happens then. Thanks for your help.